System, method and apparatus for alternator for electric machine

ABSTRACT

A stator for an electric machine includes a generally cylindrical stator core having a plurality of circumferentially-spaced and axially-extending core teeth that define a plurality of circumferentially-spaced and axially-extending core slots in a surface thereof, a main winding having a plurality of coils, each of the coils including a plurality of turns occupying the plurality of slots in the stator core, and a tertiary excitation winding having a plurality of coils, each of the coils including a single turn occupying at least a subset of the plurality of slots in the stator core. The coils of the main winding are unevenly arranged in the plurality of slots.

BACKGROUND Technical Field

Embodiments of the invention relate generally to vehicles. Certainembodiments relate to alternators for electric vehicles.

DISCUSSION OF ART

In some vehicles, such as off-highway vehicles (“OHVs”) utilized in themining industry, electrically motorized wheels propel or retard thevehicle. In particular, a large horsepower diesel engine may be usedwith an alternator, a traction inverter, and wheel drive assemblieshoused within the rear tires of the vehicle. In operation, the dieselengine drives the alternator, which powers the traction inverter. Thetraction inverter includes semiconductor power switches that commutatethe alternator output current to provide electrical power to electricdrive motors, e.g., AC traction motors, of the wheel drive assemblies,which transform the electrical power back into mechanical power to drivethe wheels and propel the vehicle.

A typical alternator utilized in electric vehicles, such as mining OHVs,is a 10-pole synchronous generator that converts engine mechanical powerto electrical power for use by the drive system of the vehicle. Thealternator stator includes a three-phase main winding to provide vehiclepropulsion power, and a single-phase tertiary winding to provideself-excitation of the alternator. Typically, the tertiary excitationwinding uses only a small portion of all the slots in a stator of thealternator, leaving a vacant space in the remaining slots that is filledwith fillers. This is inefficient, however, from an electric machinedesign perspective. Existing alternators for use in OHV applicationshave typically been designed to operate between approximately 1000V and1400V at constant power conditions.

More recent applications require the alternator to generate highervoltage, up to 1800V, in order to improve electric drive systemefficiency. In addition, efforts to improve engine efficiency, which isthe prime mover for the alternator, have led to engine speed reductions(e.g., from 1900 rpm to 1800 rpm). The combination of this highervoltage requirement and lower engine speed impose additional pressure onthe alternator, pushing the alternator to high saturation.

It has been discovered that alternators currently used in OHVapplications may not be capable of meeting these new requirements forhigher voltage and lower engine speed. Moreover, efforts to simplyincrease the number of turns within the slots of the stators, whilemeeting higher voltage requirements at reduced engine speeds, negativelyimpact operation at high current, which is critical for fuel economy ofthe vehicle.

In view of the above, there is a need for a system, method and devicethat differ from those currently available.

BRIEF DESCRIPTION

In an embodiment, a stator for an electric machine is provided. Thestator includes a generally cylindrical stator core having a pluralityof circumferentially-spaced and axially-extending core teeth that definea plurality of circumferentially-spaced and axially-extending core slotsin a surface thereof, a main winding having a plurality of coils, eachof the coils including a plurality of turns occupying the plurality ofslots in the stator core, and a tertiary excitation winding having aplurality of coils, each of the coils including a single turn occupyingat least a subset of the plurality of slots in the stator core. Thecoils of the main winding are unevenly arranged in the plurality ofslots.

In another embodiment, an alternator for an electric machine isprovided. The alternator includes a rotor having a plurality of magnetsdefining a plurality of pole pairs, a stator having a plurality ofcircumferentially-spaced and axially-extending slots, a main windinghaving a plurality of coils arranged in the plurality of slots, and atertiary excitation winding having a plurality of coils arranged in theplurality of slots. The coils of the main winding are not evenlyarranged in the plurality of slots.

In yet another embodiment, a method for improving the efficiency of anelectric machine is provided. The method includes the steps of arranginga plurality of turns of a main winding in a plurality of slots of astator of the electric machine such that a number of the turns of themain winding occupying at least one of the plurality of slots isdifferent from a number of the turns of the main winding occupying atleast another of the plurality of slots, and arranging a plurality ofturns of a tertiary excitation winding in at least a subset of slots ofthe plurality of slots.

DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a perspective view of an off-highway vehicle (OHV), accordingto an embodiment of the invention.

FIG. 2 is a schematic diagram of a drive system of the OHV of FIG. 1.

FIG. 3 is a cross-sectional view of an alternator of the drive system ofFIG. 2, according to an embodiment of the invention.

FIG. 4 is a schematic illustration of a winding arrangement or layoutfor the alternator of FIG. 3.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference characters usedthroughout the drawings refer to the same or like parts. Whileembodiments of the invention are suitable for use with vehicles,generally, an OHV has been selected for clarity of illustration for thedisclosure of mobile embodiments. Other suitable vehicles include, forexample, on-road vehicles, locomotives, construction equipment,industrial equipment, and marine vessels. Moreover, while embodimentsare described herein in connection with a 90 slot, 10-pole alternator,the invention is more generally applicable to any 18 slot per pole-pairalternator configuration.

As used herein, “electrical communication” or “electrically coupled”means that certain components are configured to communicate with oneanother through direct or indirect signaling by way of direct orindirect electrical connections. As used herein, “mechanically coupled”refers to any coupling method capable of supporting the necessary forcesfor transmitting torque between components. As used herein, “operativelycoupled” refers to a connection, which may be direct or indirect. Theconnection is not necessarily being a mechanical attachment. As usedherein, “unevenly arranged””and “not evenly arranged” means that thenumber of turns of a coil of the main winding received in at least oneof the slots of the stator is different from the number of turns of acoil of the winding received in at least another of the slots of thestator.

Embodiments of the invention relate generally to electric machines.Certain embodiments relate to system and methods or improving theefficiency of an electric machine and decreasing the fuel consumption ofa vehicle with which the electric machine is utilized. In oneembodiment, an alternator for an electric drive machine includes a rotorhaving a plurality of magnets defining a plurality of pole pairs, astator having a plurality of circumferentially-spaced andaxially-extending slots, a main winding having a plurality of coilsarranged in the plurality of slots, and a tertiary excitation windinghaving a plurality of coils arranged in the plurality of slots. The mainwinding includes coils having a different number of turns which enablesa more efficient use of slot space left from the tertiary excitationwinding. The electric drive machine with which the alternator may beused may be an off-highway vehicle or other vehicle.

An embodiment of the inventive system for improving the efficiencyand/or reducing fuel consumption is configured for use with a vehicle,such as an off-highway vehicle (“OHV”) 10 as depicted in FIG. 1. Asshown, the OHV 10 is supported on paired dual rear drive tire assemblies12 and on single front steering tire assemblies 14. The rear drive tireassemblies 12 are driven by a drive system described below in connectionwith FIG. 2.

Turning now to FIG. 2, a schematic illustration of an exemplary drivesystem 100 for an electric drive machine such as OHV 10 or other vehicleis shown. The drive system 100 includes a primary power source such asan engine 102 (e.g., a diesel engine, a gasoline engine, a multi-fuelengine, etc.) and a traction alternator/generator 104 mechanicallycoupled to and driven by the engine 102. As illustrated in FIG. 2, thetraction alternator 104 is electrically coupled to a traction bus 106.The alternator 104 is configured to provide AC electric power to one ormore rectifiers 108, which are electrically connected to one or morepower converters, e.g., first and second inverters 110, 112, via thetraction bus 106. The inverters 110, 112 are connected to first andsecond traction motors 114, 116 associated with first and second wheelsof the vehicle, e.g., rear wheels 12. (including first rear wheel 118and second rear wheel 120) of vehicle 10, respectively. The rectifier108 is configured to convert the AC power received from the alternator104 into a DC output which is then fed to the inverters 110, 112 throughthe traction bus 106. The inverters 110, 112 are configured to supplythree-phase, variable frequency AC power to the first and secondtraction motors 114, 116 associated with the first and second wheels ofthe vehicle (typically the rear wheels of the vehicle). While therectifier 108 is illustrated as being separate from the tractionalternator 104, in certain embodiments, the rectifier may form a part ofthe alternator, as is known in the art.

As also shown in FIG. 2, in an embodiment, a starter motor 122 may beassociated with the engine 102 for rotating the engine 102 so as toinitiate operation, as is known in the art. In addition, the vehicle mayinclude a battery 124, e.g. a 24V battery, electrically coupled to thealternator 104 through a tertiary winding 126 and a field winding 128.The battery 124 is configured to function as an alternator field staticexcitor to initiate operation of the electric drive system of thevehicle. Although FIG. 2 illustrates battery 120 as the excitationsource for the alternator 104, other excitation sources and arrangementsknown in the art may also be utilized without departing from the broaderaspects of the invention.

The traction motors 114, 116 provide the tractive power to move thevehicle, and may be AC or DC electric motors. When using DC tractionmotors, the output of the alternator is typically rectified to provideappropriate DC power. When using AC traction motors, the alternatoroutput is typically rectified to DC and thereafter inverted tothree-phase AC before being supplied to the traction motors 114, 116.During a propel mode of operation, power may be transferred from theengine 102 to the traction motors 114, 116, and thus to the wheels ofthe vehicle to effect movement.

In addition to providing motive power, the traction motors 114, 116 alsoprovide a braking force for controlling the speed of the vehicle onwhich the drive system 100 is deployed. This is commonly referred to asdynamic braking. During a dynamic braking mode of operation, such aswhen motion of the vehicle is to be retarded, power may be generated bythe mechanical rotation of the drive wheels and directed toward aretarding grid 130. In particular, the kinetic energy of the vehicle maybe converted into rotational power at the drive wheels. Rotation of thedrive wheels may further rotate the motors 114, 116 so as to generateelectrical power, for example, in the form of AC power. The inverters110, 112 may serve as a bridge to convert the power supplied by themotors 114, 116 into DC power. Dissipation of the DC power generated bythe motors 114, 116 may produce a counter-rotational torque at the drivewheels to decelerate the vehicle. Such dissipation may be accomplishedby passing the generated current provided by the inverters 110, 112through a resistance, such as the dynamic braking grid 130, or retardinggrid, as shown.

As further illustrated in FIG. 2, the drive system 100 may also includean engine radiator fan 132 driven by the engine 12 to provide coolingfor the engine 102. The system 100 may also include one or moreauxiliary cooling fans 134 mechanically coupled to the alternator 104.The auxiliary cooling fan(s) 134 is configured to draw air through theopening 18 of the vehicle 10 to provide cooling for other components ofthe traction drive system, such as the inverters 110, 112, tractionmotors 114, 116 and the like. The traction alternator 104 may also becoupled to a hydraulic pump 136 which provides hydraulic pressure foruse by accessories or other components of the vehicle.

In an embodiment, the drive system 100 and various components thereofmay be electrically coupled (or otherwise in communication with) andcontrolled by a controller 142. In particular, the controller 142 isconfigured to control the traction motor system 100 and the variouscomponents thereof, and the electricity supplied to and from thetraction motor system, as is known in the art. For example, thecontroller 142 is configured to control the drive system 200 to propelthe vehicle in response to a propel command received from an operator,as well as brake or slow the vehicle in response to a retard commandutilizing the drive system 100 (i.e., using the traction motors 114, 116operating in a dynamic braking mode of operation).

With reference to FIG. 3, a cross-sectional illustration of alternator104 of the drive system 100 is shown in more detail. As shown therein,the alternator 104 is a synchronous generator having a rotor 210 whichis mounted on a shaft 212 driven by the engine 102. As illustratedtherein, the rotor 210 may be permanent magnet rotor having ten magneticpoles 214 defining a plurality of pole pairs and, more particularly,five pole pairs (including, for example, pole pair 216). In otherembodiments, the rotor may be a salient-pole wound field rotor, as shownin FIG. 2.

A generally cylindrically-shaped stator core 218 surrounds the rotor210, and has a plurality of slots 220 formed in a circumferential innerdiameter 222 thereof (and spaced apart by a plurality of teeth 224). Theslots 220 extend in an axial direction and parallel to a central axis ofthe stator core 218 between a first and second end thereof. The slots220 each have a top portion 226 and a bottom portion 228. In anembodiment, there are ninety slots equally spaced around thecircumferential inner diameter 222. Each of the ninety slots 220 areindividually numbered in FIG. 3, beginning at slot 1 and proceedingclockwise to slot 90. As used here, the term “generally cylindrical”means generally having the shape of a cylinder.

As discussed in detail hereinafter, the slots 220 of the stator 218 areoccupied by a three-phase main winding having a plurality of coils thatprovide vehicle propulsion power, and a single-phase tertiary winding toprovide self-excitation of the alternator 104. Each pole pair such as,for example, pole pair 216, corresponds to a subset of slots of theplurality of slots, within which the coils of the main winding and thetertiary winding are received.

Referring now to FIG. 4, a winding layout 300 for the alternator 104 isillustrated. In the embodiment described herein, where the alternator104 has ninety slots 220 and ten poles 214 defining five pole pairs,each pole pair (e.g. pole pair 216) takes up eighteen slots 220 of thestator 218 and forms a circuit of the alternator 104. The remaining polepairs are each associated with a corresponding eighteen-slot subset ofslots having a substantially identical winding configuration. That is,the windings connected to each pole pair take up an equal fraction ofthe total number of slots of the stator 218 and have a substantiallyidentical winding layout or arrangement.

As indicated above, the stator main winding is a three-phase windinghaving phases A, B and C. In FIG. 4, slots 220, numbered 1-18, definingan exemplary eighteen-slot subset corresponding to one of the pole pairs(e.g., pole pair 216), are shown. Each of the slots 220 is occupied by aplurality of turns of coils of the main winding and/or one or more turnsof the tertiary winding. In the winding layout 300 of FIG. 4, ‘A’denotes phase A of the main winding, ‘B’ denotes phase B of the mainwinding, and ‘C’ denotes phase C of the main winding, while ‘X’ denotesphase A return, ‘Y’ denotes phase B return, and ‘Z’ denotes phase Creturn.

As shown in FIG. 4, the main winding comprises a plurality of coilshaving a different number of turns within the slots. In particular, inan embodiment, the coils of the main winding are arranged in a 5-5-6pattern in the top portion 226 of the slots, where every third slot 220(starting with slot 1) has an extra turn (i.e., 6 turns as opposed to 5turns). For example, slot 1 has 5 turns of phase A of the main winding,slot 2 has 5 turns of phase A of the main winding, and slot 3 has 6turns of phase A of the main winding. Similarly, slot 4 has 5 turns ofphase B, slot 5 has 5 turns of phase B, and slot 6 has 6 turns of phaseB. Continuing on, slot 7 has 5 turns of phase C, slot 8 has 5 turns ofphase C, and slot 9 has 6 turns of phase C. This arrangement is repeatedin slots 10-18.

As further shown in FIG. 4, the return legs of the main winding arelikewise arranged to have a different number of turns within the bottomportion 228 of the slots. For example, as shown in FIG. 4, slot 1 has 5turns of the A phase return (denoted by ‘X’), slot 2 has 6 turns of theA phase return (denoted by ‘X’) and slot 3 has 5 turns of B phase return(denoted by ‘Y’).

As shown in FIG. 4, therefore, slots 1, 4, 7, 10, 13 and 16 only haveten total turns of the main winding (including the return legs), whilethe remaining slots have eleven total turns of the main winding(including return legs). This arrangement leaves extra space in theslots having ten total turns, i.e., slots 1, 4, 7, 10, 13 and 16. Asshown in FIG. 4, this extra slot space can be occupied by a single turn312 of the tertiary winding such as, for example, in slots 1, 7, 10 and16.

As illustrated therein, each of the slots of the eighteen-slot subsettherefore has an identical number of turns (main winding plus tertiarywinding), with the exception of two slots (i.e., slots 4 and 13). Inparticular, this winding arrangement maximizes the use of the slot spacewithin the stator 218, and only leaves unoccupied space within slots 4and 13, which can be filled with a non-metallic filler material 314, asshown in FIG. 4. In an embodiment, substantially all of the slot spacein the stator 218 is occupied by turns of the main winding or tertiarywinding. In an embodiment, between approximately 80% and 100% of theslot space is occupied by the main winding and tertiary winding, leavingonly between about 0% and about 20% of the slot space occupied by fillermaterial. In yet other embodiments, approximately 90% of the slot spaceand, more particularly, about 89% of the slot space, is occupied by themain winding and tertiary winding, leaving only about 10% and, moreparticularly, about 11% of the slot space occupied by filler material.

The winding layout 300 for the 90 slot stator 218 therefore includes amain winding comprising 60 five-turn coils and 30 6-turn coils, whichoptimizes the use of the slot space. In particular, the 5-5-6 mainwinding arrangement disclosed herein uses an extra conductor in everythird slot (as compared to an adjacent and next adjacent slot), whichaffords the alternator 104 with a balanced performance capabilitybetween high voltage and high current, and meets high voltagerequirements at reduced engine speeds. In particular, the windingarrangement of the invention fills substantially all of the space in thestator slots with conductive windings (either main windings or tertiarywindings), resulting in high operating efficiency. The windingarrangement therefore improves alternator and drive system efficiency asa whole, and thereby results in improved fuel economy.

This is an improvement over existing designs which have typicallyutilized a stator winding having an identical number of turns of themain winding in each slot. In particular, existing designs typicallyfeature a main winding having 90 identical coils with 5 turns per coil.The coils of the main winding would typically be evenly installed in the90 slots of the stator core, while the tertiary winding consisting ofsingle turn coils would share and occupy 20 slots with the main winding.This layout leaves 70 slots with extra space not occupied by any turnsof coils of either the main winding or tertiary winding. This extraspace in the 70 slots is typically filled with non-metallic fillermaterial which does not assist to generate power.

The 5-5-6 winding pattern of the invention, which features an extra turnin every third slot, however, leaves only 10 slots with extra space notoccupied by any turns of coils of either the main winding or tertiarywinding. As indicated above, this arrangement provides the alternator104 with a balanced performance capability between high voltage and highcurrent, and meets high voltage requirements at reduced engine speeds.In particular, the alternator winding arrangement of the invention iscapable of generating up to approximately 1800V at engine speeds as lowas approximately 1800 rpm, yielding an improvement in drive systemefficiency over existing alternators typically designed to operatebetween about 1000V and 1400V at higher speeds of approximately 1900rpm.

In connection with the above, it has been discovered that the 5-5-6 mainwinding arrangement disclosed herein reduces stator and rotor currentdensity without changing slot shape. In particular, the improved windingarrangement disclosed herein does not require any punching change oralteration to stator core or rotor design, and can be easilyincorporated into existing stators utilizing the space that wastypically occupied by the non-metallic filler material.

As indicated above, while embodiments have been described herein inconnection with a 90 slot, 10-pole alternator, the invention is moregenerally applicable to any 18 slot per pole-pair alternatorconfiguration. For example, the winding configuration of the inventionis equally applicable to an alternator having 72 slots and eight poles(defining four pole pairs), or 54 slots and 6 poles (defining three polepairs).

In an embodiment, a stator for an electric machine is provided. Thestator includes a generally cylindrically-shaped stator core having aplurality of circumferentially-spaced and axially-extending core teeththat define a plurality of circumferentially-spaced andaxially-extending core slots in a surface thereof, a main winding havinga plurality of coils, each of the coils including a plurality of turnsoccupying the plurality of slots in the stator core, and a tertiaryexcitation winding having a plurality of coils, each of the coilsincluding a single turn occupying at least a subset of the plurality ofslots in the stator core. The coils of the main winding are unevenlyarranged in the plurality of slots. In an embodiment, a number of turnsof the main winding occupying at least one of the plurality of slots isdifferent from a number of turns of the main winding occupying at leastanother of the plurality of slots. In an embodiment, the plurality ofslots is 90 slots. In an embodiment, every third slot of the pluralityof slots has an additional turn as compared to an adjacent and a nextadjacent slot of the plurality of slots. In an embodiment, the coils ofthe main winding occupy the plurality of slots of the stator core in a5-5-6 pattern. In an embodiment, the subset of slots of the plurality ofslots occupied by the turns of the tertiary excitation winding is 20slots, and a filler material occupies 10 slots only of the plurality ofslots. In an embodiment, the main winding includes 60 5-turn coils and30 6-turn coils. In an embodiment, the electric machine is an alternatorhaving one of a permanent magnet rotor or salient-pole rotor, andincludes ten magnetic poles defining a plurality of pole pairs.

In another embodiment, an alternator for an electric machine isprovided. The alternator includes a rotor having a plurality of magnetsdefining a plurality of pole pairs, a stator having a plurality ofcircumferentially-spaced and axially-extending slots, a main windinghaving a plurality of coils arranged in the plurality of slots, and atertiary excitation winding having a plurality of coils arranged in theplurality of slots. The coils of the main winding are not evenlyarranged in the plurality of slots. In an embodiment, the plurality ofcoils of the main winding each include a plurality of turns, and theplurality of coils of the tertiary excitation winding each include asingle turn. In an embodiment, a number of the turns of the main windingoccupy at least one of the plurality of slots is different from a numberof the turns of the main winding occupy at least another of theplurality of slots. In an embodiment, the plurality of coils of the mainwinding occupy the plurality of slots of the stator in a 5-5-6 pattern.In an embodiment, the plurality of slots is 90 slots and the pluralityof magnets is 10 magnets defining 5 pole pairs. In an embodiment, themain winding includes 60 5-turn coils and 30 6-turn coils. In anembodiment, the coils of the main winding and the coils of the tertiarywinding occupy approximately 90% of a slot space defined by theplurality of slots. In an embodiment, a non-metallic filler materialoccupies a portion of not more than about 11% of the plurality of slots.In an embodiment, the electric machine is an off-highway vehicle. Andthe alternator is coupled to an engine of the off-highway vehicle.

In yet another embodiment, a method for improving the efficiency of anelectric machine is provided. The method includes the steps of arranginga plurality of turns of a main winding in a plurality of slots of astator of the electric machine such that a number of the turns of themain winding occupying at least one of the plurality of slots isdifferent from a number of the turns of the main winding occupying atleast another of the plurality of slots, and arranging a plurality ofturns of a tertiary excitation winding in at least a subset of slots ofthe plurality of slots. In an embodiment, the number of turns of themain winding occupying the at least one of the plurality of slots isgreater than the number of turns of the main winding occupying the atleast another of the plurality of slots. The subset of slots in whichthe turns of the tertiary winding are arranged includes the at leastanother of the plurality of slots. In an embodiment, the plurality ofturns of the main winding are arranged in a 5-5-6 configuration in theplurality of slots. As should be appreciated, in any of theseembodiments, the efficiency of the electric machine is improved relativeto providing the same stator but where the number of turns of the mainwinding is the same in every slot.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty.

This written description uses examples to disclose several embodimentsof the invention, including the best mode, and also to enable one ofordinary skill in the art to practice the embodiments of invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to one ofordinary skill in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1-17. (canceled)
 18. A method for improving the efficiency of anelectric machine, comprising the steps of: arranging a plurality ofturns of a main winding in a plurality of slots of a stator of theelectric machine such that a number of the plurality of turns of themain winding occupying at least one of the plurality of slots isdifferent from a number of the plurality of turns of the main windingoccupying at least another of the plurality of slots; and arranging aplurality of turns of a tertiary excitation winding in at least a subsetof slots of the plurality of slots.
 19. The method according to claim18, wherein: the number of turns of the main winding occupying the atleast one of the plurality of slots is greater than the number of turnsof the main winding occupying the at least another of the plurality ofslots; and wherein the subset of slots in which the turns of thetertiary winding are arranged includes the at least another of theplurality of slots.
 20. The method according to claim 19, wherein: theplurality of turns of the main winding are arranged in a 5-5-6configuration in the plurality of slots.
 21. The method according toclaim 19, wherein: every third slot of the plurality of slots has anadditional turn of the main winding as compared to an adjacent and nextadjacent slot of the plurality of slots.
 22. The method according toclaim 21, wherein the main winding includes 60 5-turn coils and 306-turn coils.
 23. The method according to claim 18, wherein theplurality of coils of the main winding and the plurality of coils of thetertiary excitation winding occupy approximately 90% of a slot spacedefined by the plurality of slots.
 24. The method according to claim 18,wherein at least 80% of the plurality of slots have an identical numberof turns, wherein the turns are of the main winding and/or of the mainwinding and the tertiary excitation winding.
 25. The method according toclaim 18, wherein the plurality of slots is 90 slots.
 26. The methodaccording to claim 25, wherein the subset of slots occupied by the turnsof the tertiary winding is 20 slots.
 27. The method according to claim18, wherein the electric machine is an off-highway vehicle.
 28. A methodfor decreasing fuel consumption of a vehicle comprising: arranging aplurality of magnets defining a plurality of pole pairs in a rotor;arranging a plurality of turns of a main winding in a plurality of slotsof a stator of the vehicle such that a number of the plurality of turnsof the main winding occupying at least one of the plurality of slots isdifferent from a number of the plurality of turns of the main windingoccupying at least another of the plurality of slots; and arranging aplurality of turns of a tertiary excitation winding in at least a subsetof slots of the plurality of slots; wherein the rotor is arranged insideof the stator.
 29. The method according to claim 28, wherein: a numberof the plurality of turns of the main winding occupying at least one ofthe plurality of slots is different from a number of the plurality ofturns of the main winding occupying at least another of the plurality ofslots.
 30. The method according to claim 28, wherein the main windingincludes 60 5-turn coils and 30 6-turn coils.
 31. The method accordingto claim 28, wherein the vehicle is an off-highway vehicle comprising analternator, the alternator comprising the rotor and the stator, whereinthe alternator is coupled to an engine of the off-highway vehicle.
 32. Astator for an electric machine, comprising: a generally cylindricalstator core having a plurality of circumferentially-spaced andaxially-extending core teeth that define a plurality ofcircumferentially-spaced and axially-extending core slots in a surfacethereof; a main winding having a plurality of coils, each of the coilsincluding a plurality of turns occupying the plurality ofcircumferentially-spaced and axially-extending core slots in the statorcore; and a tertiary excitation winding having a plurality of coils,each of the coils including a plurality of turns occupying at least asubset of the plurality of circumferentially-spaced andaxially-extending core slots in the stator core; wherein the pluralityof coils of the main winding is unevenly arranged in the plurality ofcircumferentially-spaced and axially-extending core slots.
 33. Thestator according to claim 32, wherein: at least 80% of the plurality ofcircumferentially-spaced and axially-extending core slots have anidentical number of turns, wherein the turns are of the main windingand/or of the main winding and the tertiary excitation winding.
 34. Thestator according to claim 32, wherein: every third slot of the pluralityof circumferentially-spaced and axially-extending core slots has anadditional turn of the main winding as compared to an adjacent and anext adjacent slot of the plurality of circumferentially-spaced andaxially-extending core slots.
 35. The stator according to claim 34,wherein: the plurality of coils of the main winding occupies theplurality of circumferentially-spaced and axially-extending core slotsof the stator core in a 5-5-6 turn pattern.
 36. The stator according toclaim 32, wherein: the number of turns of the main winding occupying theat least one of the plurality of circumferentially-spaced andaxially-extending core slots is greater than the number of turns of themain winding occupying the at least another of the plurality ofcircumferentially-spaced and axially-extending core slots; and whereinthe subset of circumferentially-spaced and axially-extending core slotsin which the turns of the tertiary winding are arranged includes the atleast another of the plurality of circumferentially-spaced andaxially-extending core slots.
 37. The stator according to claim 32,wherein: the electric machine is an alternator having one of a permanentmagnet rotor or a salient-pole rotor, and includes 18 slots perpole-pair.